Machines, including on and off-highway haul and vocational trucks, wheel loaders, motor graders, and other types of heavy machinery generally include a multi-speed, bidirectional, automatic transmission drivingly coupled to an engine by way of a hydraulic torque converter. The hydraulic torque converter multiplies and/or absorbs torque fluctuations transmitted from the engine to the transmission by allowing slippage between an output shaft of the engine and an input shaft of the transmission. Such machines also include one or more braking mechanisms, such as service brakes associated with the wheels of the machine, to controllably decelerate the machine when braking is required.
Since the engine output and transmission input shafts are mechanically coupled, the engine can be used to assist the service brakes in slowing the machine's travel. For example, during an “engine braking” operation, power can be transferred from the wheels of the machine through the transmission to drive the engine. The natural resistance of the engine may dissipate some of the transferred power, thereby slowing the machine. Moreover, during an “exhaust braking” operation, power may be dissipated by, for example, increasing backpressure in an exhaust manifold of the engine. However, because such machines typically require the use of a separate pedal or other like operator interface device to activate and control such engine or exhaust braking, it can be difficult for machine operators to utilize engine or exhaust braking, in conjunction with the service brakes of the machine, in situations where such augmented machine braking is desired. Such situations may include, for example, emergency braking, and braking while traversing a relatively steep decline or other terrain where extended machine braking is necessary.
One method of improving the retarding capacity of a machine is described in U.S. Pat. No. 6,536,408 (the '408 patent) Warner. The '408 patent describes a braking system that includes a throttle pedal position sensor, a brake pedal position switch, and a mode selector configured to operate engine compression brakes at various levels. If a sensed brake pedal position exceeds a preset position threshold, a computer associated with the mode selector may engage the engine compression brakes. The level of engine compression braking may then be increased in response to, for example, further manipulation of the brake pedal.
Although the braking system of the '408 patent may assist in controlling the travel speed of a vehicle, it may be inadequate for some situations. In particular, because the compression brakes of the '408 patent are activated solely in response to brake pedal position, the responsiveness and controllability of the system described by the '408 patent may not be acceptable in, for example, emergency braking and/or extended braking operations. For example, in such operations the compression brakes of the '408 patent may remain inactive even if engine retarding is desired by the operator.
The disclosed retarding system is directed to overcoming one or more of the problems set forth above.